Environmental control system with condition responsive timer and method

ABSTRACT

A controller (8) of air conditioning apparatus (18) with a cycle timer (12) for periodically actuating the apparatus (18) irrespective of temperature combined with a thermostat (10) for actuating the apparatus (18) in accordance with sensed temperature and logic circuitry (FIG. 3) for recycling the timer (12) to prevent excessive actuation of the apparatus (18) while ensuring minimum actuation in accordance with sensed conditions. Both the duration of the cycle period and the duty cycle of the timer are selectively variable over a wide range through means of timer circuitry employing dual electronic timers (12a and 12b) which are controlled to operate in a complementary manner.

This application is a division of application Ser. No. 779,509, filedSept. 24, 1985, now U.S. Pat. No. 4,722,475.

BACKGROUND OF THE INVENTION

This invention relates to an environmental control system, and, moreparticularly, to such a system with air conditioning apparatuscontrolled in accordance with both the condition of the environmentbeing controlled and a predetermined timing sequence.

A common method for controlling the temperature within enclosures forgrowing birds, pigs or cattle has been to use temperature responsivethermostats, such as the type that employs Freon or other gas filledcoils which expand or contract to actuate or deactuate air conditioningapparatus, such as ventilation louvers. Electronic thermostats forperforming this condition responsive control are also known. Examples ofthese are shown in U.S. Pat. No. 4,535,601 of Newell Jr. et al. entitled"Environmental Control System", filed Feb. 28, 1984, in U.S. Pat. No.4,632,304 of Newell III et al. entitled "Electronic Thermostat", filedFeb. 28, 1984 and in U.S. Pat. No. 4,534,406 of Newell III et al.entitled "Thermostat", filed June 28, 1984, all of which are assigned tothe assignee of the present application.

It is also known to use a cycle timer switch in combination with thegaseous tube-type thermostats which periodically actuate the airconditioning apparatus for a preselected portion of each preselectedtiming cycle. The duration of each cycle, or cycle period, and theportion of each cycle that the cycle timer switch actuates the airconditioning apparatus, or duty cycle, is selectively varied accordingto the age of the birds or other animals within the enclosure. This isnecessary to optimize growth and health and to compensate for increasein body heat and other factors as the animals increase in size.

In the known control systems which employ cycle timers in combinationwith condition responsive controllers, actuation of the air conditioningapparatus by the cycle timer switch is completely independent ofactuation by the condition responsive controller. Unfortunately, it istherefore not uncommon that the cycle timer will cause actuation of theair conditioning apparatus immediately after the end of actuation by thecondition responsive controller. This results in excessive actuation ofthe air conditioning apparatus that causes energy waste and erratic andexcessive temperature shifts.

Despite this problem, in the case of ventilation apparatus, the cycletimer switch control is considered necessary despite this problem toensure adequate minimum ventilation. Guaranteed adequate minimumventilation is needed for several reasons. In order to ensure safety andgood health for the animals and workers in the enclosure, the gases andmoisture caused by the products from combustion of otherwise unventedheating means and moisture and noxious gases produced from droppings andother litter must be eliminated and fresh oxygen replenished. Duringwarm periods, it is also necessary to eliminate excess heat.

It is known in other types of environments to control conditionresponsive operation in accordance with a preselected timer sequence.For instance, in U.S. Pat. No. 4,431,130 of McInnes, a timer is used todisable a radiant heat selector for a preselected time period. Likewise,in U.S. Pat. No. 4,290,480 of Sulkowski, the operation of a multiphaseenvironmental control system may be time sequenced to reduce performanceon weekends.

While these known timers have been used in other environments to goodadvantage, they fail to overcome the disadvantages of known controllersfor animal enclosures, as noted above. They lack a full range of controlover the relative duration of actuation during each cycle period.Moreover, none of these ensure a minimum amount of actuation withoutreference to sensed conditions. They therefore fail to offer solutionsto the problem of ensuring such minimum actuation while preventingexcessive time sequenced actuation. As partly noted above, excessiveactuation is wasteful of energy, interferes with accurate control of theenvironment, and has possibly harmful effects on the birds or otheranimals within the controlled environment.

SUMMARY OF THE INVENTION

Accordingly, it is the principal object of the present invention toprovide an environmental control system which overcomes theaforementioned problems of combined condition responsive and timesequenced control of air conditioning apparatus and a method ofemploying same.

This objective is achieved through provision of an environmental controlsystem having apparatus for conditioning the air within anenvironmentally controlled space with an improved controller. Theimproved controller controls actuation of the air conditioning apparatusin accordance with both the sensed condition of the environment and apreselected timing sequence. However, unlike known systems, the timesequence actuation cycle is altered in response to the sensed conditionto prevent excessive actuation while a minimum amount of actuation isstill ensured.

In a preferred embodiment of the environmental control system, animproved controller for the air conditioning apparatus is employed. Thisimproved controller comprises means to control actuation of saidapparatus in accordance with a preselected timing sequence, means forcontrolling actuation of the apparatus in response to the condition ofthe air and means responsive to the condition of the air for alteringthe timing sequence of the timing control means to minimize excessivecombined total actuation by both said timing sequence control means andsaid condition responsive controlling means. The unaltered sequence is aperiodic sequence, and the timing control means includes means foractuating the apparatus during a preselected portion of each period.Included are means for reinitiating the cycle period in response to apreselected condition occurring other than during said preselectedportion.

It is also an object of the present invention to provide such anenvironmental control system with means for selectively changing therelative duration of each of said one and the other portions of thecycle relative to the total cycle time period, or the duty cycle. Theduty cycle may be changed automatically or manually in accordance with apreselected sequence based on the animal's growth patterns.

A further objective is provision of a controller for such anenvironmental control system in which the duration of the total periodmay be selectively changed independently of any change to theaforementioned duty cycle.

Accordingly, a new method of controlling an air ventilation system of anagricultural building or the like is provided comprising the steps ofestablishing periodic timing cycles, actuating the ventilation systemfor a preselected portion of each of said time periods, sensing thecondition of the air within the building, establishing a preselectedcondition of the air within the building, establishing a preselectedtime period for each cycle, and causing the establishing means tointerrupt a current timing cycle and begin a new timing cycle inresponse to the sensed condition assuming the preselected conditionwithin the preselected time period.

It is also an object to provide a system with a timing controller inwhich the actuation portion of each cycle period is selectively variableover a full range. This objective is achieved, in part, throughprovision of an improved timing controller of the apparatus comprising apair of cycle timers, means for complementarily varying the duty cycleof both of the timers with a single control and means for actuating theapparatus during a preselected portion of the cycle of one of thetimers. Because of this unique arrangement, the duty cycles may varyover substantially the full 100% range of possible control.

cl BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects, features and advantages of the invention will bedescribed in detail and others will be made apparent from the followingdetailed description of the preferred embodiment which is given withreference to the several figures of the drawing, in which:

FIG. 1 is a functional block diagram of the environmental control systemof the present invention;

FIG. 2 is a series of waveforms illustrating the relationship betweenvarious signals generated by the controller of FIG. 1; and

FIG. 3 is a detailed circuit logic diagram of a preferred circuitimplementation of the functional block diagram of FIG. 1.

DETAILED DESCRIPTION

Referring to FIG. 1, the controller 8 is a condition responsive timerand thus includes both a condition responsive circuit 10 and a timingcircuit 12. A sequence altering logic circuit 14 receives signals fromboth the condition responsive circuit 10 and the timing circuit 12. Thelogic circuit 14 provides output signals to an actuation circuit 16 inresponse to a call for actuation from either of circuits 10 or 12 andprovides feedback signals to timing circuit 12.

The actuation circuit 16 of the controller is connected to control theactuation of an air conditioning apparatus 18. The feedback signals, onthe other hand, are employed to reset and disable the timing circuit 12depending upon the condition of its output relative to that of thecondition responsive sensor 10.

Preferably, the timing circuit 12 has means for selectively varying theduty cycle, such as a potentiometer resistor 20. The duty cycle ispreferably continuously variable between substantially a zero percentduty cycle to substantially a hundred percent duty cycle depending uponthe setting of the potentiometer resistor 20. Each cycle commences witha deactuation, or an off, portion and ends with an actuation, or an on,portion.

It is also preferred that means, such as a cycle time selection switch22, is provided to selectively establish different timing cycle timeperiods. In one position, such as the open position shown in FIG. 1, acycle time period of five minutes, for instance, is selected while in aclosed position, the timing circuit 12 has a ten minute cycle timeperiod. Other periods up to thirty minutes or more are also obtainable,as will be made apparent by reference to FIG. 3.

The condition responsive circuit comprises any appropriate sensingapparatus for sensing the condition of the air of the controlled spaceassociated with the air conditioning apparatus, such as its humidity,temperature or a combination of both and providing a binary electricaloutput signal indicative of same. The condition sensed is compared to apreselected condition, such as a set temperature. When the sensedtemperature corresponds to the preselected temperature, then thecondition responsive circuit 10 produces a sensor signal, such as a0-state signal shown in FIGS. 2-B1, 2-C1 and 2-D1, which is applied tothe input of the logic circuit 14 to indicate same. The generation ofsuch a sensor signal causes the controller 8 to actuate the airconditioning apparatus which then alters the condition of the air in thecontrolled space associated therewith.

The condition responsive sensor 10 is preferably provided with ahysterisis characteristic. Accordingly, the sensor signal continuesuntil the sensed condition assumes another preselected value differentfrom the original preselected value associated with initial actuation ofthe air conditioning apparatus.

The particular details of the condition responsive sensor form no partof this invention. However, reference may be made to U.S. Pat. No.4,535,601 of Newell Jr. et al. entitled "Environmental Control System",filed Feb. 28, 1984, to U.S. Pat. No. 4,632,304 of Newell III et al.entitled "Electronic Thermostat", filed Feb. 28, 1984, and to U.S.patent application Ser. No. 625,606 of Newell III et al. entitled"Thermostat", filed June 28, 1984, for details of a suitable circuit touse as the condition responsive sensor 10. However, with respect to thepreferred circuitry for implementation of the logic circuit 14 and theactuation circuit 16, a detailed description is provided below withreference to FIGS. 2 and 3.

The air conditioning apparatus 18 combined with the controller 8 forms acomplete environmental control system. Preferably, the air conditioningapparatus is a louvered ventilation system which increases ventilationwhen actuated, such as shown in the aforementioned patent applicationSer. No. 584,387. Alternative cooling apparatus includes condensivecooling apparatus and fans. In such case, actuation of the airconditioning apparatus causes the temperature to drop, and once it hasdropped to a preselected deactuation temperature which is less than theactuation temperature, the sensor actuation signal is terminated. In thecase of the air conditioning apparatus being a heater, the sensordeactuation temperature is greater than the preselected actuationtemperature. The air conditioning apparatus may also includehumidifiers, dehumidifiers, and water spraying and other evaporativecooling systems or any combination of these.

The relationship between the timing signal from timing circuit 12, thesensor signal from condition responsive circuit 10 and the actuationoutput signal is from actuation circuit 16 illustrated in FIG. 2. Thesesignals illustrate an 80% duty cycle of the cycle timer, in which casethere is no actuation called for until the last 20% of the cycle period.The figures of FIGS. 2-B1, 2-C1 and 2-D1 represent different sensorsignals and FIGS. 2-B2, 2-C2 and 2-D2 represent the correspondingactuation signals, respectively, presuming a normal, non-altered, timersignal as shown in FIG. 2-A1.

When a 0-state sensor signal of FIG. 2-B1 or FIG. 2-D1 is generatedduring the off period of the unaltered timer signal of FIG. 2-A, then a1-state actuation signal is generated, as shown in FIG. 2-B2 and FIG.2-D2, respectively. At the end of the sensor signal 24, the timingcircuit 12 is recycled or reset. When reset, the next actuation pulse 26does not occur when it would if there were no sensor signal 24, as shownin FIG. 2-1A. Instead, it does not begin until the end of a complete offperiod TF of the duty cycle, as shown in FIGS. 2-B2 and 2-D2.

If sensor actuation signal 24 both starts and stops during an offperiod, as shown in FIG. 2-B1, then an actuation signal 24' will begenerated during this same time period.

If the sensor signal 24 commences during the on portion of the dutycycle and continues beyond the end of this on portion of the timersignal of FIG. 2-A1, as shown in FIG. 2-D1, then the actuation signal 30commences with the on portion of timer signal of FIG. 2-A1 and continuesuntil the end of the 0-state sensor signal 32, as shown in FIG. 2-D2. Atthe end of the sensor signal 32, the timer cycle again begins, or isrecycled, and an actuation signal 34 is generated at the end of a fulloff period TF of the timer cycle, as shown in Fig. D2.

If the sensor signal both begins before and ends after the on portion ofa timer signal, then the actuation signal coincides with the sensorsignal. At the end of the sensor signal, the timer commences a new cycleand the next actuation signal is generated in response to the sensorsignal at the end of a full off period TF, in the same manner, as shownin FIG. 2-D2.

If a sensor signal 36 begins after the start of a timer signal of FIG.2-A and ends before the end of the timer signal, as shown in FIG. 2-C1,then the actuation signal 38 is uneffected and continues the same as ifthe sensor signal did not occur, as shown in FIG. 2-C2.

Referring now to FIG. 3, a detailed functional block diagram and circuitlogic diagram of a preferred embodiment of the controller 8 of FIG. 1 isshown. When the temperature gets warmer, then the sensor signal willincrease in duration, such as the sensor signal actuation pulse, or ONpulse, 32 of FIG. 2-D1 is increased relative to sensor signal pulse 36of FIG. 2-C1 or relative to the pulse 24 of FIG. 2-B1. In such case, theactuation period of actuation circuit 16 of FIG. 3 is increased andcooling is increased. The timing circuit 12 includes an off timer 12Aand an on timer 12B which may be substantially identical to one anotherand which function in a complementary fashion. Use of conventionalintegrated circuit timers, such as an LM 555 or LM 556 made by NationalSemiconductor have been found suitable for this function.

Each has a control input 40A and 40B which are connected to thejunctions 42A and 42B, respectively, at the opposite sides of apotentiometer resistor 20 and a cycle selection switch 22. Cycleselection switch 22 has two sections 22A and 22B respectively associatedwith a pair of capacitors 44A and 46A and a pair of capacitors 44B and46B. These capacitors are connected between opposite sides of theirassociated switch section and ground.

When the switch 22 is in the closed position, as shown in FIG. 3, thenthe total capacitance of both associated capacitors is connected withthe potentiometer resistor 20 and the associated control input toestablish a first cycle time period on the order of sixty seconds forthe associated timer. When the switch 22 is in an open position,capacitors 46A and 46B are disconnected from the potentiometer. Thisreduces the total capacitance connected with the potentiometer andthereby reduces the associated RC time period and the cycle time of bothtimers 12A and 12B.

The potentiometer slide contact is connected with a positive DC powersupply voltage V obtained from a DC power supply 48. When power is firstapplied, the capacitors 44A and 46A begin to charge to voltage V throughthe associated section 20A of potentiometer resistor 20, and arelatively small current limiting resistor 50A. Resistor 50A has a valuewhich is less than one percent of that of the potentiometer resistor 20,so that an approximately 100% range of control of duty cycle isobtainable. Likewise, when power is first applied, capacitors 44B and46B are charged through the associated section 20B of potentiometerresistor 20 and a limiting resistor 50B.

As noted above, the off timer 12A and on timer 12B operate in acomplementary fashion. This is achieved through the symmetrical andcommon connection of the potentiometer with both timers. This connectionis symmetrical since all the corresponding circuit elements associatedwith opposite sides of the potentiometer resistor 20 are equal to oneanother.

Capacitors 46A and 46B are equal to one another and in one embodimenthave a capacitance value on the order of 6.8 microfarad and capacitors44 and 44B are of equal value on the order of 8.3 microfarads. Likewise,resistors 41A and 41B are of equal value of approximately ten kilohm.Potentiometer resistor has a value on the order of five megaohms whichis one hundred times larger than the total of limit resistors 50A and50B.

The total of the time constants of the two RC circuits respectivelyassociated with the on and off timers 12A and 12B remains constant andis established solely by the value of the capacitors and the total valueof the three resistors 50A, 20 and 50B. This value does not change withchanges in the potentiometer setting.

However, the individual RC time constants of off timer 12A and on timer12B, which determine the duty cycle, vary with variations in thepotentiometer setting. They are therefore linearly related to oneanother. As the time constant of one increases by a given percentage ofthe total time constant, the other decreases by that same percentage.

When the potentiometer is set at the center of the potentiometer, asshown, then resistor portions 20A and 20B are equal and the two RC timerconstants are equal. Accordingly, a 50% duty cycle will be established.However, for purposes of discussion, it will be presumed that thepotentiometer slider is at a 20% duty cycle setting and the cycle timeselection switch has been set for a five minute cycle time. In thatevent, in a normal, uninterrupted cycle, the cycle timer will place theair conditioning apparatus in a deactuated state for the first 80% ofthe total cycle time, i.e., for the first four minutes of the fiveminute cycle. It will then cause the apparatus to assume an actuatedstate for the next remaining 20% of the cycle period, or for one minute.This cycle will then be repeated.

Initially, both timers 12A and 12B are reset by a reset circuit 50 whichmaintains a 0-state reset signal on its output 50A for a preselectedtime period after DC power voltage V is applied to the remainder of thecontroller circuit. Preferably, it comprises a single RC time delaycircuit connected between supply voltage V and ground referencepotentiometer.

This 0-state reset pulse is used to reset the timers 12A and 12B and twocounters, or dividers, off counter 56 and on counter 58 respectivelyassociated therewith. First, a 0-state reset pulse is applied directlyto a reset input 59 of on timer 12B. This pulse is also inverted by aninverter. 53 to produce a 1-state reset pulse on its output 52A. This0-state reset pulse is applied via a connection lead 54 to a reset input56A of the on counter 56. It is also applied to the reset input 58A.This 1-state reset pulse is also applied to an input of a logic OR-gate78. The OR-gate 78 responds by producing a 1-state reset pulse on itsoutput which is applied via a connection 62 to the input of an inverter64. Inverter 64 inverts this to a 0-state reset pulse which is appliedto a reset input 66 of off timer 12A.

In this reset condition, all the circuitry of controller 8 is in acondition which causes the air conditioning apparatus 18 to be in adeactuated state. The details of the actuation circuit form no part ofthe present invention, and reference may be made to the aforementionedU.S. patent application Ser. No. 584,398 for details of suitablecircuitry useful for this purpose. Briefly, the the actuation circuit 16has a relay or electronic switch which closes to short circuit a pair ofoutput terminals 16A and 16B in response to a 1-state signal at itsinput 16C. This shorting of the output terminals causes actuation of theair conditioning apparatus 18 connected therewith. In the case of anautomatic ventilation system, ventilation would be increased upon beingactuated as compared to when deactuated.

The logic actuation signal applied to the input 16C of the actuationcircuit is taken from the output 60C of a logic OR-gate 60. OR-gate 60generates a logic 1-state actuation signal whenever either of its inputs60A and 60B is in a 1-state. One of the inputs 60A is taken from thecondition responsive sensor 10 via a resistor 68 and an inverter 70. Theother input is taken from the output 12C of timing circuit 12. Thus,whenever either the condition responsive sensor 10 calls for actuationthrough generation of a 0-state sensor signal or the timing circuitcalls for actuation through generation of a 1-state timer actuationsignal on its output 12A, a logic 1-state actuation signal is generatedon the output 60C of OR-gate 60.

The sequence altering logic circuit includes a logic OR-gate 72 havingone input 72A connected to the output of the condition responsive sensor10 through the resistor 68. The other input 72B is connected to theoutput 58B of the off counter 58 of timing circuit 12, and the output72C is connected to the junction 42B of resistor 50B and capacitors 44Band 46B through means of a diode 74.

Thus, whenever both a 0-state sensor signal is being generated and a0-state off counter output signal is being generated, the output ofOR-gate 72 will switch to a 0-state sequence alteration signal. Wheneverthis alteration signal is generated, it causes the capacitors 44A, 46A,44B and 46B to be discharged to ground reference potential through thediode 74. This disables the on counter 12B from generating further1-state output pulses to trigger on counter 56. Accordingly, the countof on counter 56 remains the same and its output remains in the off1-state condition. This output is inverted to a 0-state by an inverter76 and then applied to the input of an OR-gate 78. The resultant 0-stateoutput signal from OR-gate 78 is first inverted to a 1-state signal byinverter 64 and then applied to the reset input 66 of off timer 12A. Theoff timer 12A is thereby reset and when the condition responsive signalis terminated the timing cycle is begun anew.

For purposes of timing, a trigger pulse is also applied to trigger input56B of the on counter 56 from the output 58B of off counter 58 throughan AC coupling capacitor 83 and a diode 84. Likewise, the output of oncounter 56 is connected to off counter 58 through a capacitor 89 anddiode 87, while the output of off timer 12A is coupled to the input ofoff counter 58 through a capacitor 90 and a diode 88.

A further feature of the invention is an anti-lock-up circuit whichincludes an OR-gate 85 to generate appropriate reset pulses. The OR-gatehas its output connected through an isolation diode 86 to the resetinput 59 of on timer 12B. It is also connected to the start-up resetoutput 50A at the juncture with inverter 52 to apply a reset pulse to oncounter 56, off counter 58 and off timer 12A, as previously explainedwith reference to operation of the start up reset circuit 50. Thus, whenthe output of OR-gate 85 generates a 0-state reset pulse, all countersand timers are reset and operation resumes in the same way as when poweris first applied, as described above.

The OR-gate 85 has two inputs. One input is taken from the output 52A ofinverter 52 to receive signals from start-up reset circuit. The otherinput is taken from the output 56C of on counter 56.

The anti-lock-up circuit operates to prevent a lock-up of all circuitswhich could result from a noise pulse causing both the on counter 56 andthe off counter 58 to be on at the same time. If this occurs, thenlock-up condition is sensed by the OR-gate 85 which generates 1-stateanti-lock-up pulse on its output. This anti-lock-out pulse then resetsall the circuits as described above, so that normal operation canresume.

While a preferred embodiment has been disclosed for purposes ofillustration, the scope of the invention is not so limited, but insteadis defined in the following claims. For instance, while in theparticular embodiment, the timer is disabled from actuating the airconditioning apparatus for a period to time equal to its off cycleperiod, such disablement period could last longer if desired.

We claim:
 1. In an environmental control system having apparatus forconditioning the air within an environmentally controlled space, animproved time controller of the apparatus comprising:a pair of cycletimers; means for complementarily varying the duty cycles of both ofsaid timers with a single control, manual movement of said controlincreasing the duty cycle of one of said timers and substantiallyreducing the timing cycle of the other by a substantially equal amountrelative to the total cycle period; and means for actuating theapparatus during a preselected portion of the cycle of one of saidtimers.
 2. The system of claim 1 including means for substantiallyselectively changing the effective value of capacitors connected withsaid pair of timers by substantially equal amounts to change their cycleperiods by equal amounts.
 3. The system of claim 2 in which said cycleperiod changing means includes a double sectioned switch connected withsaid capacitors.
 4. The system of claim 1 including means for disablingat least one of said timers in response to sensed condition of thespace.
 5. The system of claim 1 in whichsaid cycle timers alternatelychange between opposite states, and the respective states of said pairof cycle timers during normal operation have a preselected relationshipwith one another.
 6. The system of claim 5 including an anti-lock-upcircuit for preventing said timers from remaining in respective stateswhich are not in said preselected relationship with one another.
 7. Thesystem of claim 6, in which said anti-lock-up circuit comprisesmeans fordetecting when the cycle timers are in other than said preselectedrelationship, and means responsive to said detecting means for resettingsaid timers when other than said preselected relationship is detected.8. The system of claim 6 in which said detecting means comprises a logicgate with two inputs respectively coupled to the outputs of said pair ofcycle timers, respectively, and an output coupled to reset inputs ofeach of said cycle timers.
 9. The system of claim 6 in which saidpreselected relationship exists when the pair of timers are in oppositestates.
 10. In an environmental control system having apparatus forconditioning the air within an environmentally controlled space, animproved time controller of the apparatus, comprising:a pair of cycletimers; means for complementarily varying the duty cycles of both ofsaid timers with a single control; means for actuation the apparatusduring a preselected portion of the cycle of one of said timers; and apair of counters respectively triggered by said pair of timers toenlarge the cycle periods associated therewith.
 11. In an environmentalcontrol system having apparatus for conditioning the air within anenvironmentally controlled space, an improved time controller of theapparatus, comprising:a pair of cycle timers; means for complementarilyvarying the duty cycles of both of said timers with a single control,said single control included a potentiometer resistor symetricallyconnected with both of said timers; and means for actuating theapparatus during a preselected portion of the cycle of one of saidtimers.
 12. In an environmental control system having apparatus forconditioning the air within an environmentally controlled space, animproved time controller of the apparatus comprising:a pair of cycletimers; means for complementarily varying the duty cycles of both ofsaid timers with a single control, said single control including apotentiometer resistor symetrically connected with both of said timersand a pair of substantially equal compacitors connected with oppositesides of said potentiometer resistor and to said pair of timers,respectively; and means for actuating the apparatus during a preselectdportion of the cycle of one of said timers.